Methods and apparatus for making holograms

ABSTRACT

A method and apparatus for making holograms includes a technique for exposing a film substrate or other light-sensitive medium to consecutive two-dimensional images, together representative of a physical three-dimensional system, to generate a three-dimensional hologram of the physical system. Low beam ratios are employed to superimpose multiple (20-300) images on the substrate. Each image is relatively weak, but the combination of the series of weak images ultimately appears as a single clearly defined hologram.

This is a continuation of application Ser. No. 07/982,316 filed Nov. 27,1992, which is herewith abandoned.

TECHNICAL FIELD

The present invention relates, generally, to methods and apparatus formaking holograms, and more particularly to a technique for sequentiallyexposing a film substrate to a plurality of two-dimensional imagesrepresentative of a three-dimensional physical system to thereby producea hologram of the physical system.

BACKGROUND ART AND TECHNICAL PROBLEMS

A hologram is a three-dimensional record, e.g., a film record of aphysical system which, when replayed, produces a true three-dimensionalimage of the system. Holography differs from stereoscopic photography inthat the holographic image exhibits full parallax by affording anobserver a full range of viewpoints of the image from every angle, bothhorizontal and vertical, and full perspective, i.e. it affords theviewer a full range of perspectives of the image from every distancefrom near to far. A holographic representation of an image thus providessignificant advantages over a stereoscopic representation of the sameimage. This is particularly true in medical diagnosis, where theexamination and understanding of volumetric data is critical to propermedical treatment.

While the examination of data which fills a three-dimensional spaceoccurs in all branches of art, science, and engineering, perhaps themost familiar examples involve medical imaging where, for example,Computerized Axial Tomography (CT or CAT), Magnetic Resonance (MR), andother scanning modalities are used to obtain a plurality ofcross-sectional images of a human body part. Radiologists, physicians,and patients observe these two-dimensional data "slices" to discern whatthe two-dimensional data implies about the three-dimensional organs andtissue represented by the data. The integration of a large number oftwo-dimensional data slices places great strain on the human visualsystem, even for relatively simple volumetric images. As the organ ortissue under investigation becomes more complex, the ability to properlyintegrate large amounts of two-dimensional data to produce meaningfuland understandable three-dimensional mental images may becomeoverwhelming.

Other systems attempt to replicate a three-dimensional representation ofan image by manipulating the "depth cues" associated with visualperception of distances. The depth cues associated with the human visualsystem may be classified as either physical cues, associated withphysiological phenomena, or psychological cues, which are derived bymental processes and predicated upon a person's previous observations ofobjects and how an object's appearance changes with viewpoint.

The principal physical cues involved in human visual perception include:(1) accommodation (the muscle driven change in focal length of the eyeto adapt it to focus on nearer or more distant objects); (2) convergence(the inward swiveling of the eyes so that they are both directed at thesame point); (3) motion parallax (the phenomenon whereby objects closerto the viewer move faster across the visual field than more distantobjects when the observer's eyes move relative to such objects); and (4)stereo-disparity (the apparent difference in relative position of anobject as seen by each eye as a result of the separation of the twoeyes). The principal psychological cues include: (1) changes in shading,shadowing, texture, and color of an object as it moves relative to theobserver; (2) obscuration of distant objects blocked by closer objectslying in the same line of sight; (3) linear perspective (a phenomenonwhereby parallel lines appear to grow closer together as they recedeinto the distance); and (4) knowledge and understanding which is eitherremembered or deduced from previous observations of the same or similarobjects.

The various psychological cues may be effectively manipulated to createthe illusion of depth. Thus, the brain can be tricked into perceivingdepth which does not actually exist. However, the physical depth cuesare not subject to such manipulation; the physical depth cues, whilegenerally limited to near-range observation, accurately conveyinformation relating to an image. For example, the physical depth cuesare used to perceive depth when looking at objects in a small room. Thepsychological depth cues however, must be employed to perceive depthwhen viewing a photograph or painting (i.e. a planar depiction) of thesame room. While the relative positions of the objects in the photographmay perhaps be unambiguously perceived through the psychological depthcues, the physical depth cues nonetheless continue to report that thephotograph or painting is merely a two-dimensional representation of athree-dimensional space.

Stereo systems depend on image pairs each produced at slightly differentperspectives. The differences in the images are interpreted by thevisual system (using the psychological cues) as being due to relativesize, shape, and position of the objects and thus create the illusion ofdepth. A hologram, on the other hand, does not require the psychologicalcues to overrule the physical depth cues in order to create the illusionof a three-dimensional image; rather, a hologram produces an actualthree-dimensional image.

Conventional holographic theory and practice teach that a hologram is atrue three-dimensional record of the interaction of two beams ofcoherent, i.e. mutually correlated light, in the form of a microscopicpattern of interference fringes. More particularly, a reference beam oflight is directed at the film substrate at a predetermined angle withrespect to the film. An object beam, which is either reflected off of orshines through the object to be recorded, is generally normally(orthogonally) incident to the film. The reference and object beamsinteract within the volume of space occupied by the film and, as aresult of the coherent nature of the beams-, produce a standing (static)wave pattern within the film. The standing interference patternselectively exposes light sensitive elements within the photographicemulsion comprising the film, resulting in a pattern of alternatinglight and dark lines known as interference fringes. The fringe pattern,being a product of the standing wave front produced by the interferencebetween the reference and object beams, literally encodes the amplitudeand phase information of the standing wave front. When the hologram isproperly re-illuminated, the amplitude and phase information encoded inthe fringe pattern is replayed in free space, producing a truethree-dimensional image of the object.

Conventional holographic theory further suggests that a sharp, welldefined fringe pattern produces a sharp, bright hologram, and that anoverly strong object beam will act like one or more secondary referencebeams causing multiple fringe patterns to form (intermodulation) anddiluting the strength of the primary fringe pattern. Accordingly,holographers typically employ a reference beam having an amplitude atthe film surface approximately five to eight times that of the objectbeam to promote the formation of a single high contrast pattern withinthe interference fringe pattern and to reduce spurious noise resultingfrom bright spots associated with the object. Moreover, since knownholographic techniques generally surround the recording of a singlehologram or, alternatively, up to two or three holograms, within asingle region of the emulsion comprising film substrate, the statedobjective is to produce the strongest fringe pattern possible to ensurethe brightest holographic display. Accordingly, holographers typicallyattempt to expose a large number of photosensitive grains within thefilm emulsion while the object is being exposed. Since every pointwithin the holographic film comprises part of a fringe pattern whichembodies information about every visible point on the object, fringepatterns exist throughout the entire volume of the film emulsion,regardless of the configuration of the object or image which is thesubject of the hologram. Consequently, the creation of strong, highcontrast fringe patterns necessarily results in rapid consumption of thefinite quantity of photosensitive elements within the emulsion, therebylimiting the number of high contrast holograms which can be produced ona single film substrate to two or three. Some holographers havesuggested that as many as 10 to 12 different holographic imagestheoretically may be recorded on a single film substrate; however,heretofore, superimposing more than a small finite number of hologramshas not been recognized and, in fact, has not been possible in thecontext of conventional hologram theory.

In prior art holograms employing a small number of superimposedholographic images on a single film substrate, the existence of arelatively small percentage of spurious exposed and/or developedphotosensitive elements (fog) does not appreciably degrade the qualityof the resulting hologram. In contrast, holograms made in accordancewith the subject invention, discussed below, typically employ up to 100or more holograms superimposed on a single film substrate; hence, thepresence of a small amount of fog on each hologram would have a seriouscumulative effect on the quality of the final product.

A method and apparatus for producing holograms is therefore needed whichpermits a large number, for example several hundred or more differentholograms, to be recorded on a single film substrate, therebyfacilitating the true, three-dimensional holographic reproduction ofhuman body parts and other physical systems which are currently viewedin the form of discrete data slices.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for makingholograms which overcome the limitations of the prior art.

In accordance with one aspect of the present invention, a hologramcamera assembly comprises a single laser source and a beam splitterconfigured to split the laser beam into a reference beam and an objectbeam and to direct both beams at a film substrate. The assembly furthercomprises a spatial light modulator configured to sequentially project aplurality of two-dimensional images, for example a plurality of slicesof data comprising a CT scan data set, into the object beam and onto thefilm. In this manner, a three-dimensional holographic record of eachtwo-dimensional slice of the data set is produced on the film.

In accordance with another aspect of the invention, the entire data set,consisting of one to two hundred or more individual two-dimensionalslices, is superimposed onto the film, resulting in the superposition ofone hundred or more individual, interrelated holograms on the singlesubstrate (the master hologram). In contrast to prior art techniqueswherein a small number (e.g., one to four) of holograms are superimposedonto a single film substrate, the present invention contemplates methodsand apparatus for recording a large number of relatively weak holograms,each consuming an approximately equal, but in any event proportionate,share of the photosensitive elements within the film.

In accordance with a further aspect of the invention, a copy (transfer)assembly is provided whereby the aforementioned master hologram may bequickly and efficiently reproduced in a single exposure as a singlehologram.

In accordance with yet a further aspect of the invention, a reference toobject beam ratio of approximately unity is employed in making themaster hologram, thereby conserving the number of photo-sensitiveelements (e.g., silver halide crystals) which are usefully converted foreach two-dimensional data slice. Moreover, careful control over variousprocess parameters, including the coherence, polarization, andscattering of the laser beam, as well as the exposure time and the greylevel value of the data, permit each individual hologram comprising themaster hologram to consume (convert) a quantity of silver halidecrystals within the emulsion in proportion to, among other things, thenumber of data slices comprising the data set.

In accordance with yet a further aspect of the invention, a hologramviewing device is provided for viewing the hologram produced inaccordance with the invention. In particular, an exemplary viewing boxin accordance with the present invention comprises a suitably enclosed,rectangular apparatus comprising a broad spectrum light source, e.g., awhite light source mounted therein, a collimating (e.g., Fresnel) lens,a diffraction grating, and a Venetian blind (louver). The collimatinglens is configured to direct a collimated source of white light throughthe diffraction grating. In the context of the present invention, acollimated light refers to light in which all components thereof havethe same direction of propagation such that the beam has a substantiallyconstant cross-sectional area over a reasonable propagation length.

The diffraction grating is configured to pass light therethrough at anangle which is a function of the wavelength of each light component. Thehologram also passes light therethrough at respective angles which are afunction of the corresponding wavelengths. By inverting the hologramprior to viewing, all wavelengths of light thus emerge from the hologramsubstantially orthogonally thereto. .

DRAWING FIGURES

The subject invention will hereinafter be described in conjunction withthe appended drawing figures, wherein like numerals denote likeelements, and:

FIG. 1A shows a typical computerized axial tomography (CT) device;

FIG. 1B shows a plurality of two-dimensional data slices each containingdata such as may be obtained by x-ray devices typically employed in theCT device of FIG. 1A, the slices cooperating to form a volumetric dataset;

FIG. 1C shows an alternative volumetric data set obtained through use ofan angled gantry;

FIG. 1D shows yet another volumetric data set such as is typicallyobtained from an ultrasound device;

FIG. 2A sets forth a conventional HD graph for typical holographic filmsamples;

FIG. 2B sets forth a graph of diffraction efficiency as a function ofbias energy in accordance with one aspect of the present invention;

FIG. 3 shows a schematic diagram of a camera system in accordance with apreferred embodiment of the present invention;

FIG. 4 shows a schematic diagram of a beam splitter assembly inaccordance with a preferred embodiment of the present invention;

FIG. 5A to 5D are graphic illustrations showing the effect of Fouriertransforming of the laser beam utilized in the camera system of FIG. 3;

FIG. 6 shows an enlarged schematic diagram of a portion of the camerasystem of FIG. 3;

FIG. 7 shows an enlarged schematic diagram of another portion of thecamera system of FIG. 3;

FIG. 8 shows an enlarged schematic diagram of a portion of theprojection assembly utilized in the camera assembly of FIG. 3;

FIG. 9 shows a schematic layout of an exemplary copy rig in accordancewith the present invention;

FIGS. 10A and 10B set forth orthoscopic and pseudoscopic views,respectively, of a master hologram being replayed in accordance with oneaspect of the present invention;

FIG. 11 shows a schematic diagram of a hologram viewing apparatus; and

FIGS. 13A-D schematically illustrate fringe patterns associated withtransmission and reflection holograms, respectively.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

In the context of the present invention, a volumetric data setcorresponding to a three-dimensional physical system (e.g., a human bodypart) is encoded onto a single recording material, e.g., photographicsubstrate, to thereby produce a master hologram of the object. Themaster hologram may be used to produce one or more copies which, whenreplayed by directing an appropriate light source therethrough,recreates a three-dimensional image of the object exhibiting fullparallax and full perspective. Thus, for a particular data set, thepresent invention contemplates a plurality of separate, interrelatedoptical systems: a camera system for producing a master hologram; a copysystem for generating copies of the master hologram; and a viewingsystem for replaying either the master hologram or copies thereof,depending on the particular configuration of the camera system.

THE DATA SET

Presently known modalities for generating volumetric data correspondingto a physical system include, inter alia, computerized axial tomography(CAT or CT) scans, magnetic resonance scans (MR), three-dimensionalultra sound (US), positron emission tomography (PET), and the like.Although a preferred embodiment of the present invention is describedherein in the context of medical imaging systems which are typicallyused to investigate internal body parts (e.g., the brain, spinal cord,and various other bones and organs), those skilled in the art willappreciate that the present invention may be used in conjunction withany suitable data set defining any three-dimensional distribution ofdata, regardless of whether the data set represents a physical system,e.g., numerical, graphical, and the like.

Referring now to FIGS. 1A-D, a typical CT device comprises a gantry 10and a table 12, as is known in the art. Table 12 is advantageouslyconfigured to move axially (along arrow A in FIG. 1) at predeterminedincrements. A patient (not shown) is placed on table 12 such that thebody part to be interrogated is generally disposed within the perimeterof gantry 10.

Gantry 10 suitably comprises a plurality of x-ray sources and recordingdevices (both not shown) disposed about its circumference. As thepatient is moved axially relative to gantry 10, the x-ray devices recorda succession of two-dimensional data slices 14A, 14B, . . . 14Xcomprising the three-dimensional space (volume) 16 containing dataobtained with respect to the interrogated body part (see FIG. 1B). Thatis, the individual data slices 14 combine to form a volumetric data set16 which, in total, comprises a three-dimensional image of theinterrogated body part. As used herein, the terms "volume" or"volumetric space" refers to volumetric data set 16, including aplurality of two-dimensional data slices 14, each slice containingparticular data regarding the body part interrogated by the givenmodality.

Typical data sets comprise on the order of 10 to 70 (for CT systems) or12 to 128 (for MR) two-dimensional data slices 14. Those skilled in theart will appreciate that T/the thickness and spacing between data slices14 are configurable and may be adjusted by the CT technician. Typicalslice thicknesses range from 1.5 to 10 millimeters and most typicallyapproximately 5 millimeters. The thickness of the slices is desirablyselected so that only a small degree of overlap exists between eachsuccessive data slice.

The data set corresponding to a CT or MR scan is typically reproduced inthe form of a plurality (e.g., 50-100) of two-dimensional transparentimages which, when mounted on a light box, enable the observer (e.g.,physician) to view each data sluice. By reviewing a plurality ofsuccessive data slices 14, the observer may construct athree-dimensional mental image or model of the physical system withinvolume 16. The accuracy of the three-dimensional model constructed inthe mind of the observer is a function of the level of skill,intelligence, and experience of the observer and the complexity anddegree of abnormality of the body parts within volume 16.

In certain circumstances it may be desirable to tilt gantry 10 about itshorizontal axis B such that the plane of gantry 10 forms a preselectedangle, for example angle α, with respect to the axis of travel of table12 for some or all data slices. With particular reference to FIG. 1C,use of an angled gantry produces a data set corresponding to analternate volume 18 comprising a plurality of data slices 18A, 18B, 18C,. . . 18X, where X corresponds to the number of data slices and whereinthe plane of each data slice forms an angle (α) with respect to axis A.In circumstances where the interrogated body part is adjacent to aradiation sensitive physiological structure (e.g., the eyes), the use ofan angled gantry permits data to be gathered without irradiating theclosely proximate radiation sensitive material.

In addition to the use of an angled gantry, other techniques may beemployed to produce a data set in which a plane of each data slice isnot necessarily parallel to the plane of every other data slice, or notnecessarily orthogonal to the axis of the data set; indeed, the axis ofthe data set may not necessarily comprise a straight line. For example,certain computerized techniques have been developed which artificiallymanipulate the data to produce various perspectives and viewpoints ofthe data, for example, by graphically rotating the data. In suchcircumstances, it is nonetheless possible to replicate thethree-dimensional data set in the context of the present invention. Inparticular, by carefully coordinating the angle at which the object beamis projected onto the film, the plane of a particular data slice may beproperly oriented with respect to the plane of the other data slices andwith respect to the axis of the data set, for example by tilting screen326 about its horizontal or vertical axis.

With momentary reference to FIG. 1D, a typical ultra sound data set 20comprises a plurality of data slices 20A, 20B, 20C . . . 20X defining afan-out volumetric space.

Virtually any suitable volumetric configuration may be defined by a dataset in the context of the present invention. Thus, while each data slicemay not necessarily be parallel to every other data slice comprising aparticular data set, fairly accurate images may be produced providedeach data slice is substantially parallel to its adjacent slice.Further, those skilled in the are will know that computer programs canbe used to reformat data sets to provide parallel slices in planes otherthan the acquisition plane of the scanner.

Presently known CT scan systems generate data slices having a resolutiondefined by, for example, a 256 or a 512 square pixel matrix.Furthermore, each address within the matrix is typically defined by atwelve bit grey level value. CT scanners are conventionally calibratedin Houndsfield Units whereby air has a density of minus 1,000 and watera density of zero. Thus, each pixel within a data slice may have a greylevel value between minus 1,000 and 3,095 (inclusive) in the context ofa conventional CT system. Because the human eye is capable ofsimultaneously perceiving a maximum of approximately one hundred (100)grey levels between pure white and pure black, it is desirable tomanipulate the data set such that each data point within a sliceexhibits one (1) of approximately fifty (50) to one hundred (100) greylevel values (as opposed to the 4,096 available grey level values). Theprocess of redefining these grey level values is variously referred toas "windowing" (in radiology); "stretching" (in remote sensing/satelliteimaging); and "photometric correction" (in astronomy).

The present inventor has determined that optimum contrast may beobtained by windowing each data slice in accordance with its content.For example, in a CT data slice which depicts a cross-section of a bone,the bone being the subject of examination, the relevant data willtypically exhibit grey level values in the range of minus 600 to 1,400.Since the regions of the data slice exhibiting grey level values lessthan minus 600 or greater than 1,400 are not relevant to theexamination, it may be desirable to clamp all grey level values above1,400 to a high value corresponding to pure white, and those data pointshaving grey level values lower than minus 600 to a low valuecorresponding to pure black.

As a further example, normal grey level values for brain matter aretypically in the range of about 40 while grey level values correspondingto tumorous tissue may be in the 120 range. If these values wereexpressed within a range of 4,096 grey level values, it would beextremely difficult for the human eye to distinguish between normalbrain and tumorous tissue. Therefore, it may be desirable to clamp alldata points having grey level values greater than, e.g., 140, to a veryhigh level corresponding to pure white, and to clamp those data pointshaving grey scale values of less than, e.g., minus 30, to a very lowvalue corresponding to pure black. Windowing the data set in this mannercontributes to the production of sharp, unambiguous holograms.

In addition to windowing a data set on a slice-to-slice basis, it mayalso be advantageous, under certain circumstances, to performdifferential windowing within a particular slice, i.e. from pixel topixel. For example, a certain slice or series of slices may depict adeep tumor in a brain, which tumor is to be treated with radiationtherapy, for example by irradiating the tumor with one or more radiationbeams. In regions which are not to be irradiated, the slice may bewindowed in a relatively dark manner. In regions which will have low tomid levels of radiation, a slice may be windowed somewhat more brightly.In regions of a high concentration of radiation, the slice may bewindowed even brighter. Finally, in regions actually containing thetumor, the slice may be windowed the brightest. In the context of thepresent invention, the resulting hologram produces a ghostly image ofthe entire head, a brighter brain region, with the brightest regionsbeing those regions which are either being irradiated (if the data setwas taken during treatment) or which are to be irradiated.

Another step in preparing the data set involves cropping, wherebyregions of each data slice or even an entire data slice not germane tothe examination are simply eliminated. Cropping of unnecessary data alsocontributes to the formation of sharp, unambiguous holograms.

More particularly, each point within the volume of the emulsion exhibitsa microscopic fringe pattern corresponding to the entire holographicimage from a unique viewpoint. Stated another way, an arbitrary point atthe lower left hand corner of a holographic film comprises aninterference fringe pattern which encodes the entire holographic imageas the image is seen from that particular point. Another arbitrary pointon the holographic film near the center of the film will comprise aninterference fringe pattern representative of the entire holographicimage when the image is viewed from the center of the film. These samephenomena hold true for every point on the hologram. As brieflydiscussed above, a suitable photographic substrate preferably comprisesa volume of photographic emulsion which adheres to the surface of aplastic substrate, for example triacetate. The emulsion typicallycomprises a very large number of silver halide crystals (grains)suspended in a gelatinous emulsion. Inasmuch as the emulsion contains afinite quantity of crystals, the elimination of unnecessary data(cropping) within a data slice ensures that substantially all of thesilver halide grains which are converted (exposed) for each data slicecorresponding to the relevant data from each slice. By conserving thenumber of silver halide grains which are converted for each data slice,a greater number of slices may be recorded onto a particular piece offilm.

THE CAMERA SYSTEM

Once a data set is properly prepared (e.g., windowed and cropped), anindividual hologram of each respective data slice is superimposed onto asingle film substrate to generate a master hologram. In accordance witha preferred embodiment, each individual hologram corresponding to aparticular data slice is produced while the data corresponding to aparticular slice is disposed at a different distance from the filmsubstrate, as discussed in greater detail below.

Referring now to FIGS. 3-4, a camera system 300 in accordance with thepresent invention suitably comprises a laser light source 302, a shutter306, a first mirror 308, a beam splitting assembly 310, a second mirror312, a reference beam expander 314, a collimating lens 316, a filmholder 318, a third mirror 320, an object beam expander 322, an imagingassembly 328, a projection optics assembly 324, a rear projection screen326 comprising a diffusing surface 472 having a polarizer 327 mountedthereto, and a track assembly 334. Imaging assembly 328, projectionoptics assembly 324, and rear projection screen 326 are suitably rigidlymounted to track assembly 334 so that they move in unison as trackassembly 334 is moved axially along the line indicated by arrow F. Asdiscussed in greater detail below, track assembly 334 is advantageouslyconfigured to replicate the relative positions of data slices comprisingthe subject of the hologram. In a preferred embodiment, total travel oftrack assembly 334 is suitably sufficient to accommodate the actualtravel of the particular scanning modality employed in generating thedata set, for example on the order of 6 inches.

Camera assembly 300 is illustratively mounted on a rigid table 304 whichis suitably insulated from environmental vibrations. In particular, theinterference fringe pattern created by the interaction between theobject beam and the reference beam is a static wave front which hasencoded therein phase and amplitude information about the "object" whichis the subject of the hologram. Any relative motion between the objectbeam, reference beam, and the film within which the hologram is recordedwill disrupt this static interference pattern, resulting in significantdegradation of the recorded hologram. Thus, it is important that theentire camera assembly be isolated from external vibrations.

To achieve vibration isolation, table 304 suitably comprises a rigidhoneycomb top table, e.g., a RS series RS-512-18 product manufactured byNewport of Irvine, Calif. Table 304 is suitably mounted on a plurality(e.g., 4) of pneumatic isolators, e.g., Stabilizer I-2000 alsomanufactured by Newport.

As an alternative to pneumatically isolating the camera assembly fromexternal vibrations, the various components (including table 304)comprising the camera assembly, may be made from rigid materials andsecurely mounted to table 304. Such a highly rigid system, whilenonetheless vulnerable to a certain degree of externally or internallyimposed vibration is likely to move as a single rigid body in responseto such vibrations, and can be designed so that it tends to dampenrelative motion between the various parts of the system.

To compensate for the low amplitude vibration which inevitably affectsthe assembly, a technique known as "fringe locking" may be employed.More particularly, the fringe pattern exhibited at the film upon whichthe hologram is recorded may be magnified and observed by one or morephoto diodes (a typical fringe pattern exhibits alternating regions ofdark and clear lines). To compensate for any motion of the fringepattern detected by the photo diode, the pathlength of either thereference beam or the object beam may be manipulated to maintain astable fringe pattern. For this purpose, a suitable component, forexample, one of the mirrors used to direct the object beam or thereference beam, may be mounted on a piezoelectric element configured tomove slightly in a predetermined direction in accordance with a voltagesignal applied to the piezoelectric element. The output of the photodiode may be applied to a servo-loop which, when applied to thepiezoelectric element upon which the mirror is mounted, rapidly correctsthe pathlength to compensate for motion of the fringe pattern as sensedby the photo diode. In this way, although small amplitude relativemotion between the various components comprising the camera assembly maynonetheless exist, it may be compensated for in the foregoing manner.

Laser source 302 suitably comprises a conventional laser beam generator,for example an Argon ion laser including an etalon element to reduce thebandwidth of the emitted light, preferably an Innova 306-SF manufacturedby Coherent, Inc. of Palo Alto, Calif. Those skilled in the art willappreciate that L/laser 302 suitably generates a monochromatic beamhaving a wavelength in the range of 400 to 750 nanometers (nm), andpreferably about 514.5 or 532 nm. Those skilled in the art willappreciate, However, that any suitable wavelength may be used for whichthe selected photographic material is compatible, including wavelengthsin the ultraviolet and infrared ranges.

Alternatively, laser 302 may comprise a solid state, diode-pumpedfrequency-doubled YAG laser, which suitably emits laser light at awavelength of 532 nm, These lasers are capable of emitting in the rangeof 300 to 600 milliwatts of pure light, are extremely efficient andair-cooled, and exhibit high stability.

Laser 302 should also exhibit a coherence length which is at least asgreat as the difference between the total path traveled by the referenceand object beams, and preferably a coherence length of at least twicethis difference. In the illustrated embodiment, nominal design pathlength traveled by the reference beam is equal to that of the objectbeam (approximately 292 centimeters); however, due to, inter alia, thegeometry of the set-up, the particular reference angle employed, and thesize of film, some components of the reference and object beams maytravel a slightly greater or lesser path length. Hence, laser 302exhibits a coherence length in excess of this difference, namelyapproximately two meters.

Shutter 306 suitably comprises a conventional electromechanical shutter,for example a Uniblitz model no. LCS4Z manufactured by VincentAssociates of Rochester, N.Y. In a preferred embodiment, shutter 306 maybe remotely actuated so that a reference beam and an object beam areproduced only during exposure of the film substrate, effectivelyshunting the laser light from the system (e.g., via shutter 306) at allother times. Those skilled in the art will appreciate that T/the use ofa shutter is unnecessary if a pulse laser source is employed. Moreover,it may be desirable to incorporate a plurality of shutters, for examplea shutter to selectively control the reference beam and a differentshutter to separately control the object beam, to permit independentcontrol of each beam, for example to permit independent measurementand/or calibration of the respective intensities of the reference andobject beams at the film surface.

The various mirrors (e.g., first mirror 308, second mirror 312, thirdmirror 320, etc.) employed in camera assembly 300 suitably compriseconventional front surface mirrors, for example a dielectric mirrorcoated on a pyrex substrate, for example stock mirror 10D20BD.1,manufactured by Newport. For a typical laser having a beam diameter onthe order of 1.5 millimeters, mirror 308 suitably has a surface ofapproximately 1 inch in diameter.

First mirror 308 is suitably configured to direct a source beam 402 tobeam splitting assembly 310. In the illustrated embodiment, first mirror308 changes the direction of beam 402 by 90 degrees. Those skilled inthe art, however, will appreciate that the relative disposition of thevarious optical components comprising camera assembly 300, and theparticular path traveled by the various beams, are in large measure afunction of the physical size of the available components. As a workingpremise, it is desirable that the reference beam and object beam emanatefrom the same laser source to ensure proper correlation between thereference and the object beam at the surface of film holder 318, andthat the path traveled by the reference beam from beam splitter 310 tofilm 319 is approximately equal to the path traveled by the object beamfrom beam splitter 310 to film 319.

With momentary reference to FIG. 4, beam splitter assembly 310preferably comprises a variable wave plate 404, respective fixed waveplates 408 and 412, respective beam splitting cubes 406 and 414, and amirror 416. On a gross level, beam splitting assembly 310 functions toseparate source beam 402 into an object beam 410 and a reference beam418. Moreover, again with reference to FIG. 3, beam splitter assembly310 also cooperates with imaging assembly 328 and polarizer 327 toensure that the reference beam and the object beam are both purelypolarized in the same polarization state, i.e., either purely S orpurely P polarized as discussed in greater detail below, when theycontact an exemplary film substrate 319 mounted in film holder 318. Byensuring that the reference and object beams are pure polarized in thesame polarization state, sharp, low noise interference fringe patternsmay be formed.

With continued reference to FIG. 4, beam 402 generated by laser source302 enters beam splitting assembly 310 in a relatively pure polarizationstate, for example as S polarized light. In the context of the presentinvention, S polarized light refers to light which is polarized with itselectric field oscillating in a vertical plane; P polarized light refersto light having its electric field oriented in a horizontal plane. Beam402 then passes through variable wave plate 404 whereupon the beam isconverted into a beam 403, conveniently defined as comprising a mixtureof S and P polarized light components. Beam 403 then enters beamsplitting cube 406, which is suitably configured to split beam 403 intoa first beam 405 comprising the P polarized light component of beam 403and a second beam 407 comprising the S polarized light component of beam403. Beam splitting cube 406 suitably comprises a broad band beamsplitter, for example a broad band polarization beam splitter, part no.05FC16PB.3, manufactured by Newport. Although beam splitting cube 406 isideally configured to pass all of (and only) the P polarized componentof beam 403 and to divert all of (and only) the S polarized component of403, it has been observed that such cubes are generally imperfect beamsplitters, ignoring small losses due to reflection off of beam splittersurfaces. More precisely, such cubes typically exhibit an extinctionratio on the order of a thousand to one such that approximately 99.9percent of the S polarized component of beam 403 is diverted into beam407, and such that approximately 90 percent of the P polarized componentof beam 403 passes through cube 406. Thus, beam 407 comprises 99.9percent of the S polarized component of beam 403, and approximately 10percent of the P polarized component of beam 403; similarly, beam 405comprises approximately 90 percent of the P polarized component of beam403 and approximately 0.1 percent of the S polarized component of beam403.

Wave plates 404, 408, and 412 suitably comprise half wave plates for thelaser wavelength in use, e.g., part no. 05RP02 available from Newport.Wave plate 404 is configured to convert the S polarized beam 402 into apredetermined ratio of S and P polarized components. In a preferredembodiment, variable wave plate 404 comprises an LCD layer, which layerchanges the polarization of the incoming beam in accordance with thevoltage level at the LCD layer. A suitable wave plate 404 may comprise aLiquid-Crystal Light Control System, 932-VIS manufactured by Newport.Accordingly, wave plate 404 divides S polarized beam 402 into a mixtureof S and P polarized light as a function of applied voltage. Bymanipulating the voltage on wave plate 404, the operator therebycontrols the ratio of the intensity of the reference beam to theintensity of the object beam (the beam ratio). In a preferredembodiment, this ratio as measured at the plane of film 319 isapproximately equal to unity.

In any event, beam 405 is almost completely pure P polarized, regardlessof the voltage applied to wave plate 404; beam 407 is ideally pure Spolarized, but may nonetheless contain a substantial P polarizedcomponent, depending on the voltage applied to wave plate 404.

With continued reference to FIG. 4, beam 405 then travels through waveplate 408 to convert the pure P polarized beam 405 to a pure S polarizedobject beam 410. Beam 407 is passed through wave plate 412 to convertthe substantially S polarized beam to a substantially P polarized beam409 which thereafter passes through splitting cube 414 to eliminate anyextraneous S component. In particular, 99.9 percent of the residual Scomponent of beam 409 is diverted from cube 414 as beam 415 and shuntedfrom the system. In the context of the present invention, any beam whichis shunted from or otherwise removed from the system may be convenientlyemployed to monitor the intensity and quality of the beam.

The predominantly P component of beam 409 is passed through cube 414 andreflected by respective mirrors 416 and 312, resulting in asubstantially pure P polarized reference beam 418. As discussed ingreater detail below, by dividing source beam 402 into object beam 410and reference beam 418 in the foregoing manner, both the object beam andreference beam exhibit extremely pure polarization, for example on theorder of one part impurity in several thousand. Moreover, a high degreeof polarization purity is obtained regardless of the beam ratio, whichis conveniently and precisely controlled by controlling the voltageapplied to variable wave plate 404.

With continued reference to FIGS. 3 and 4, beam 418 is reflected offmirror 312 and enters beam expander 314. Beam expander 314 preferablycomprises a conventional positive lens 421 and a tiny aperture 420. Thediameter of beam 418 at the time it enters beam expander 314 is suitablyon the order of approximately 1.5 millimeters (essentially the samediameter as when it was discharged from laser 302). Positive lens 421 isconfigured to bring beam 418 to as small a focus as practicable. Asuitable positive lens may comprise microscope objective M-20Xmanufactured by Newport. Aperture 420 suitably comprises a pin-holeaperture, for example a PH-15 aperture manufactured by Newport. For goodquality lasers which emit pure light in the fundamental transverseelectromagnetic mode (TEM₀₀), a good quality lens, such as lens 421, cantypically focus beam 418 down to the order of approximately 10 to 15microns in diameter. At the point of focus, the beam is then passedthrough aperture 420, which suitably comprises a small pin hole on theorder of 15 microns in diameter. Focusing the beam in this mannereffects a Fourier transform of the beam.

More particularly and with reference to FIGS. 5A-5D, the TEM₀₀ mode ofpropagation typically exhibited by a small diameter laser beam follows aGaussian distribution transverse to the direction of propagation of thebeam. With specific reference to FIG. 5A, this means that the intensity(I) of beam 418 exhibits a Gaussian distribution over a cross-section ofthe beam. For a Gaussian beam having a nominal diameter of onemillimeter, a small amount of the beam at very low intensity extendsbeyond the 1 millimeter range.

With reference to FIG. 5B, a more accurate representation of the idealcondition shown in FIG. 5A illustrates a substantially Gaussiandistribution, but also including the random high frequency noiseinevitably imparted to a beam as it is bounced off mirrors, polarized,etc. Note that FIG. 5B exhibits the same basic Gaussian profile of thetheoretical Gaussian distribution of FIG. 5A, but further includingrandom high frequency noise in the form of ripples.

It is known that the Fourier transform of a Gaussian with noise producesthe same basic Gaussian profile, but with the high frequency noisecomponents shifted out onto the wings, as shown in FIG. 5C. When theFourier transform of the beam is passed through an aperture, such asaperture 420 of beam expander 314, the high frequency wings are clipped,resulting in the extremely clean, noise free Gaussian distribution ofFIG. 5D. Quite literally, focusing the beam to approximate a pointsource, and thereafter passing it through an aperture has the effect ofshifting the high frequency noise to the outer bounds of the beam andclipping the noise.

Beam expander 314 thus produces a substantially noise free, Gaussiandistributed divergent reference beam 423.

In a preferred embodiment of the present invention, lens 421 andaperture 420 suitably comprise a single, integral optical component, forexample a Spatial Filter 900 Model manufactured by Newport. Beamexpander assembly 314 advantageously includes a screw thread, such thatthe distance between lens 421 and aperture 420 may be preciselycontrolled, for example on the order of about 5 millimeters, and twoorthogonal set screws to control the horizontal and vertical positionsof the aperture relative to the focus of lens 421.

With continued reference to FIG. 3, mirror 312 is suitably configured todirect beam 423 at film 319 at a predetermined angle which closelyapproximates Brewster's angle for the material comprising film 319.Those skilled in the art will appreciate that Brewster's angle is oftendefined as the arc tangent of the refractive index of the material uponwhich the beam is incident (here, film 319). Typical refractive indicesfor such films are in the range of approximately 1.5±0.1. Thus, inaccordance with a preferred embodiment of the invention, mirror 312 isconfigured such that beam 423 strikes film 319 at a Brewster's angle ofapproximately 56 degrees (arc tan 1.5≅56 degrees). Those skilled in theart will also appreciate that a P polarized beam incident upon a surfaceat Brewster's angle will exhibit minimum reflection from that surface,resulting in maximum refraction of reference beam 423 into film 319,thereby facilitating maximum interference with the object beam andminimum back-reflected light which could otherwise eventually find itsway into the film from an incorrect direction.

Referring now to FIGS. 4 and 6-7, object beam 410 is reflected by mirror320 and directed into beam expander 322 which is similar in structureand function to beam expander 314 described above in conjunction withFIG. 4. A substantially noise free, Gaussian distributed divergentobject beam 411 emerges from beam expander 322 and is collimated by acollimating lens 434, resulting in a collimated object beam 436 having adiameter in the range of approximately 5 centimeters. Collimating lens434 suitably comprises a bi-convex optical glass lens KBX148manufactured by Newport. Collimated object beam 436 is applied toimaging assembly 328.

With reference to FIGS. 7 and 8, imaging assembly 328 suitably comprisesa cathode ray tube (CRT) 444, a light valve 442, a wave plate 463, and apolarizing beam splitting cube 438. In a preferred embodiment, beamsplitting cube 438 is approximately a 5 centimeter square (2 inchsquare) cube. As discussed in greater detail below, a beam 460,comprising a P polarized beam which incorporates the data from a dataslice through the action of imaging assembly 328, emerges fromprojection assembly 328 and is applied to imaging optics assembly 324.

As discussed above, a data set comprising a plurality of two-dimensionalimages corresponding to the three-dimensional subject of the hologram isprepared for use in producing the master hologram. The data set may alsobe maintained in an electronic data file in a conventional multi-purposecomputer (not shown). The computer interfaces with CRT 444 such that thedata slices are transmitted, one after the other, within imagingassembly 328.

More particularly, a first data slice is projected by CRT 444 onto lightvalve 442. As explained in greater detail below, the image correspondingto the data slice is applied onto film 319. The reference and objectbeams are applied to film 319 for a predetermined amount of timesufficient to permit film 319 to capture (record) a fringe patternassociated with that data slice and thereby create a hologram of thedata slice within the emulsion comprising film 319. Thereafter, trackassembly 334 is moved axially and a subsequent data slice is projectedonto film 319 in accordance with the distances between data slices; asubsequent hologram corresponding to the subsequent data slice is thussuperimposed onto film 319. This process is sequentially repeated foreach data slice until the number of holograms superimposed onto film 319corresponds to the number of data slices 14 comprising the particularvolumetric data set 16 which is the subject matter of the masterhologram being produced.

More particularly and with continued reference to FIGS. 7 and 8, CRT 444suitably comprises a conventional fiber-optic face-plate CRT, forexample, H1397T1 manufactured by the Hughes Aircraft Company ofCarlsbad, Calif. CRT 444 is configured to project an image correspondingto a particular data slice onto the left hand side of light valve 442(FIG. 7).

In a preferred embodiment, light valve 442 is a Liquid Crystal LightValve H4160 manufactured by Hughes Aircraft Company of Carlsbad, Calif.With specific reference to FIG. 8, light valve 442 preferably comprisesa photocathode 454, a mirror 450, having its mirrored surface facing tothe right in FIG. 8, and a liquid crystal layer 452. Liquid crystallayer 452 comprises a thin, planar volume of liquid crystal which altersthe polarization of the light passing therethrough as a function of thelocalized voltage level of the liquid crystal.

Photocathode 454 comprises a thin, planar volume of a photovoltaicmaterial which exhibits localized voltage levels as a function of lightincident thereon. As the image corresponding to a particular data slice14 is applied by CRT 444 onto photocathode 454, local photovoltaicpotentials are formed on the surface of photocathode 454 in directcorrespondence to the light distribution within the cross-section of theapplied image beam. In particular, the beam generated by CRT 444corresponding to the data slice typically comprises light regionscorresponding to bone, soft tissue, and the like, on a dark background.The dark background areas predictably exhibit relatively low grey scalevalues, whereas the lighter regions of the data slice exhibitcorrespondingly higher grey scale values. A charge distributioncorresponding to the projected image is produced on the surface ofphotocathode 454.

The static, non-uniform charge distribution on photocathode 454,corresponding to local brightness variations in the data embodied in aparticular data slice 14, passes through mirror 450 and producescorresponding localized voltage levels across the surface of liquidcrystal layer 452. These localized voltage levels within liquid crystallayer 452 rotate the local liquid crystal in proportion to the localvoltage level, thereby altering the pure S polarized light diverted fromcube 438 onto mirrored surface 450, into localized regions of polarizedlight having a P component associated therewith, as the light passesthrough crystal layer 452 and is reflected by mirror 450. The emergingbeam 460 exhibits (in cross-section) a distribution of P polarized lightin accordance with the voltage distribution within crystal layer 452and, hence, in accordance with the image corresponding to the thencurrent data slice 14.

Substantially all (e.g., 99.9%) of the S polarized light comprising beam436 is diverted by cube 438 onto liquid crystal layer 452. This Spolarized light is converted to P polarized light by liquid crystallayer 452 in accordance with the voltage distribution on its surface, asdescribed above. The P polarized light is reflected by the mirroredsurface of mirror 450 back into cube 438; the P polarized light passesreadily through cube 438 into projection optics assembly 324.

The S component of the beam reflected off of the mirrored surface ofmirror 450 will be diverted 90 degrees by beam splitting cube 438. Toprevent this stray S polarized light from re-entering the system, cube438 may be tilted slightly so that this S polarized light is effectivelyshunted from the system.

The resultant beam 460 exhibits a distribution of P polarized lightacross its cross-section which directly corresponds to the data embodiedin the data slice currently projected by CRT 444 onto light valve 442.As a result of the high extinction ratio of cube 438, beam 460 comprisesessentially zero S polarization. Note also that the small portion of Spolarized light comprising beam 436 which is not reflected by cube 438into light valve 442 (namely, a beam 440) may be conveniently shuntedfrom the system.

Beam splitting cube 438 is similar in structure and function to beamsplitting cubes 406 and 414, described herein in connection with FIG. 4,and preferably comprises a large broad band polarization beam splitter,for example a PBS-514.5-200 manufactured by CVI Laser Corporate ofAlbuquerque, N. Mex. In a preferred embodiment, beam splitting cube 438has a cross-section at least as large as the image projected by CRT 444onto light valve 442, e.g., 2 inches. This is in contrast to beamsplitting cubes 406 and 414 which can advantageously be of smallercross-section, e.g., one-half inch, comparable to the diameter of theunexpanded beam 402 from laser 302.

In the context of the present invention, light which is variouslydescribed as removed, eliminated, or shunted from the system may bedisposed of in any number of convenient ways. For example, the light maybe directed into a black box or onto a black, preferably texturedsurface. The precise manner in which the light is shunted, or theparticular location to which the light is shunted is largely a matter ofconvenience; what is important is that light is to be removed from thesystem be prevented from striking the film surface of a hologram (forreasons discussed herein), and further that the light be prevented fromreentering the laser source which could disturb or even damage thelaser.

Although imaging optics 328 illustratively comprises light valve 442,any suitable mechanism which effectively integrates the imagecorresponding to a data slice into the object beam will work equallywell in the context of the present invention. Indeed, light beam 460,after emerging from cube 438, merely comprises a nonuniform distributionof P polarized light which varies in intensity according to thedistribution of data on the then current data slice 14. Thecross-section of beam 460 is substantially identical to a hypotheticalbeam of P polarized light passed through a photographic slide of theinstant data slice.

With continued reference to FIGS. 7 and 8, wave plate 463 is suitablyinterposed between light valve 442 and beam splitting cube 438. Waveplate 463 functions to correct certain undesirable polarization whichlight valve 442 inherently produces.

More particularly, light valve 442 polarizes the light which passesthrough liquid crystal layer 452 in accordance with the local voltagedistribution therewithin. Specifically, the applied voltage causes theliquid crystals to rotate, e.g., in an elliptical manner, the amount ofrotation being proportional to the localized voltage level. That is, avery high voltage produces a large amount of liquid crystal rotation,resulting in a high degree of alteration of the polarization of thelight passing through the rotated crystals. On the other hand, a verylow voltage produces a correspondingly small degree of liquid crystalrotation, resulting in a correspondingly small amount of alteration inthe level of the polarization. However, it has been observed that a verysmall degree of liquid crystal rotation (pre-tilt) exists even in theabsence of an applied voltage. Thus, approximately one percent of the Spolarized light passing through liquid crystal layer 452 is converted toP polarized light, even within local regions of liquid crystal layer 452where no voltage is applied. While this very small degree of spuriouspolarization does not generally degrade the performance of light valve442 in most contexts, it can be problematic in the context of thepresent invention. For example, if one percent of pure S polarized lightis inadvertently converted to P polarized light, the contrast ratio ofthe resulting hologram may be substantially limited.

Wave plate 463 is configured to compensate for the foregoing residualpolarization by, for example, imparting a predetermined polarization tothe light passing therethrough, which is calculated to exactly cancelthat amount of polarization induced by liquid crystal layer 452 in theabsence of an applied voltage. By eliminating this undesiredpolarization, the effective contrast ratio of the resulting hologram islimited only by the degree of control achieved in the various processparameters, as well as the inherent capabilities of the equipmentcomprising camera assembly 300.

With reference to FIGS. 6 and 7, projection optics assembly 324 suitablycomprises a projection lens 462, a mirror 464, and an aperture 466. Lens462 preferably comprises a telecentric projection lens optimized forspecific image sizes used on light valve 442 and rear projection screen326. Lens 462 converges collimated beam 460 until the converging beam,after striking mirror 464, converges to a focal point, whereupon itthereafter forms a divergent beam 470 which effectively images the datacorresponding to the then current data slice 14 onto projection screen326 and onto film 319. Beam 470 passes through an aperture 466 atapproximately the point where beam 470 reaches a focal point. Aperture466 preferably comprises an iris diaphragm ID-0.5 manufactured byNewport. Note, however, that aperture 466 is substantially larger thanthe diameter of beam 470 at the point where the beam passes throughaperture 466. This is in contrast to the pinhole apertures comprisingbeam expanders 314 and 322 which function to remove the high frequencycomponents from the beam. The high frequency components within beams 460and 470 are important in the present invention inasmuch as they maycorrespond to the data which is the subject of the hologram beingproduced. Aperture 466 simply traps and shunts scattered light andotherwise misdirected light carried by beam 470 or otherwise visible toprojection screen 326 and which is not related to the informationcorresponding to the data on data slice 14.

With reference to FIGS. 3, 4 and 6, beam 470 is projected to apply afocused image onto rear projection screen 326. Screen 326 is suitably onthe order of 14 inches in width by 12 inches in height, and preferablycomprises a thin, planar diffusing material adhered to one surface of arigid, transparent substrate, for example a 0.5 inch thick glass sheet472. Diffuser 472 is fabricated from a diffusing material, e.g.,Lumiglas-130 manufactured by Stewart Filmscreen Corporation of Torrance,Calif. Screen 472 diffuses beam 470 such that each point within beam 470is visible over the entire surface area of film 319. For example, anexemplary point Y on beam 470 is diffused by diffuser 472 so that theobject beam at point Y manifests a conical spread, indicated by cone Y',onto film 319. Similarly, an arbitrary point X on screen 472 casts adiffuse conical projection X' onto film 319. This phenomenon holds truefor every point within the projected image as the image passes throughdiffuser 472. As a result, every point on film 319 embodies a fringepattern which encodes the amplitude and phase information for everypoint on diffuser 472.

Since light from every point on diffuser 472 is diffused onto the entiresurface of film 319, it follows that every point on film 319 "sees" eachand every point within the projected image as the projected imageappears on diffuser 472. However, each point on film 319 necessarilysees the entire image, as the image appears on diffuser 472, from aslightly different perspective. For example, an arbitrary point Z onfilm 319 "sees" every point on diffuser 472. Moreover, an arbitrarypoint W on film 319 also "sees" every point on screen 472, yet from avery different perspective than point Z. Thus, after emerging fromdiffuser 472 and polarizer 327, the diffuse image carried by object beam473 is applied onto film 319.

Polarizer 327 is advantageously mounted on the surface of diffusingdiffuser 472. Although the light (beam 470) incident on diffusingdiffuser 472 is substantially P polarized, diffuser 472, by its verynature, scatters the light passing therethrough, typically depolarizingsome of the light. Polarizer 327, for example a thin, planer, polarizingsheet, repolarizes the light so that it is in a substantially pure Ppolarization state when it reaches film 319. Note that polarizer 327 isdisposed after diffuser 472, so that the light improperly polarized bydiffuser 472 is absorbed. This ensures that a high percentage of theobject beam, being substantially purely P polarized, will interfere withthe reference beam at film 319, further enhancing the contrast of eachhologram.

The manner in which the complex object wave front traveling fromdiffuser 472 to film 319 is encoded within the film, namely in the formof a static interference pattern, is the essence of holographicreproduction. Those skilled in the art will appreciate that theinterference (fringe) pattern encoded within the film is the result ofconstructive and destructive interaction between the object beam and thereference beam. That being the case, it is important that the objectbeam and reference beam comprise light of the same wavelength. Althoughtwo light beams of different wavelengths may interact, the interactionwill not be constant within a particular plane or thin volume (e.g., the"plane" of the recording film). Rather, the interaction will be atime-varying function of the two wavelengths.

The static (time invariant) interaction between the object and referencebeams in accordance with the present invention results from themonochromatic nature of the source of the reference and object beams(i.e. monochromatic laser source 302 exhibiting an adequate coherencelength). Moreover, those skilled in the art will further appreciate thatmaximum interaction occurs between light beams in the same polarizationstate. Accordingly, maximum interaction between the object and referencebeams may be achieved by ensuring that each beam is purely polarized inthe same polarization state at the surface of film 319. For theconfiguration set forth in FIG. 6, the present inventor has determinedthat P polarized light produces superior fringe patterns. Thus, toenhance the interference between object beam 470 and reference beam 423,beam 470 passes through polarizing screen 327 adhered to the surface ofdiffuser 472.

The pure P polarized reference beam 423 passes through a collimatinglens 316 and is collimated before striking film 319. Inasmuch as thereference and object beams both emanate from the same laser 302, andfurther in view of the relatively long coherence length of laser 302relative to the differential path traveled by the beams from the laserto film 319, the reference and object beams incident on film 319 aremutually coherent, monochromatic (e.g., 514.5 nm highly purely Ppolarized) and, hence, highly correlated. In addition, reference beam423 is highly ordered, being essentially noise free and collimized.Object beam 470, on the other hand, is a complicated wave front whichincorporates the data from the current data slice. These two wavesinteract extensively within the volume of the emulsion comprising film319, producing a static, standing wave pattern. The standing wavepattern exhibits a high degree of both constructive and destructiveinterference. In particular, the energy level E at any particular pointwithin the volume of the emulsion may be described as follows:

    E≅[A.sub.o Cos β.sub.o +A.sub.r Cos β.sub.r ].sup.2

where A_(o) and A_(r) represent the peak amplitude of the object andreference beams, respectively, at a particular point, and β_(o) andβ_(r) represent the phase of the object and reference beams at that samepoint. Note that since the cosine of the phase is just as likely to bepositive as negative at any given point, the energy value E at any givenpoint will range from 0 to 4A² (A_(o) =A_(r) for a unity beam ratio).This constructive and destructive wave interference produces welldefined fringe patterns.

With momentary reference to FIG. 12, the relative orientation of thereference beam, object beam, and replay beam is illustrated in thecontext of a transmission hologram (FIGS. 12A and 12B) and a reflectionhologram (FIGS. 12C and 12D).

The emulsion within which a fringe pattern is recorded is typically onthe order of about six microns in thickness. With particular referenceto FIG. 12A, alternating black and white lines of a fringe patterntypically span the emulsion much like the slats of a venetian blind,parallel to a line bisecting the angle between the reference beam (RB)and object beam (OB). When the transmission hologram shown in FIGS. 12Aand 12B is replayed with a replay beam (PB), the fringe planes act likepartial mirrors, observer 32 thus views a transmission hologram from theopposite side from which the replay beam is directed.

In a reflection hologram, on the other hand, the fringe lines aresubstantially parallel to the plane of the film (FIGS. 12C and 13C).Those skilled in the art will appreciate that reflection holograms aretypically produced by directing the reference beam and the object beamsfrom opposite sides of the film. When a reflection hologram is replayed,the replay beam (PB) is directed from the same side from which thereference beam (RB) was directed, resulting in a reflection of thereplay beam (PB) along the direction of the original object beam (OB).While many aspects of the present invention may be employed in thecontext of a reflection hologram, the apparatus and methods describedherein are best suited for use in conjunction with transmissionholograms. Moreover, it can be appreciated that transmission hologramsare less sensitive to vibration during manufacture, inasmuch as thefilms are typically mounted in a vertical plane, and a high percentageof spurious environmental vibrations comprise a large horizontalcomponent.

With continued reference to FIG. 12A, the object beam (OB) and referencebeam (RB) form a record of a microscopic fringe pattern within theemulsion in the form of alternating dark and clear lines. The darkregions generally correspond to relatively high localized energy levelssufficient to convert silver halide crystals and thus create a record ofthe interference pattern.

For each data slice, film 319 will be exposed to the standing wavepattern for a predetermined exposure time sufficient to convert thatdata slice's pro rata share of silver halide grains.

After film 319 is exposed to the interference pattern corresponding to aparticular data slice, track assembly 334 is moved forward (or,alternatively, backward) by a predetermined amount proportional to thedistance between the data slices. For example, if a life size hologramis being produced from CT data, this distance suitably correspondsexactly to the distance traveled by the subject (e.g., the patient) atthe time the data slices were generated. If a less than or greater thanlife size hologram is being produced, these distances are variedaccordingly.

In accordance with a preferred embodiment of the invention, film 319suitably comprises HOLOTEST (™) holographic film, for example Film No.8E 56HD, manufactured by AGFA, Inc. The film suitably comprises agelatinous emulsion prepared on the surface of a plastic substrate. Anexemplary film may have a thickness on the order of 0.015 inches, withan emulsion layer typically on the order of approximately 6 microns.

During the early 1980s, commercial holographic films were primarily madeusing a plastic substrate comprising polyester, principally because ofits superior mechanical properties (tear resistance, curl resistance,resistance to fading, etc.) However, typical polyesters exhibit a degreeof birefringence, i.e. the P components of the incident beam travelthrough the material at a different rate (and hence a differentdirection) than the S components. For holograms recorded or using anunpolarized source, e.g., a white light source, various componentswithin the white light travel through the material in differentdirections, resulting in compromised fidelity of the replayed hologram.As a result, the industry now generally employs a non-birefringenttriacetate substrate because of its minimal affect on the polarizationof incident light.

In accordance with one aspect of the present invention, both thereference beam and object beam incident on the holographic film, whetherduring production of the master hologram or during production of thecopy hologram, is substantially pure polarized. That being the case, thebirefringent property of polyester does not adversely affect the subjectholograms. Moreover, in transmission holography, the reference andobject beams may be configured to interact at the emulsion before eitherbeam reaches the substrate; hence, birefringence is less of a problemfor this reason also. Accordingly, holographic films used in the contextof the present invention typically comprise a polyester backing, therebyexploiting the superior mechanical properties of the film without thedrawbacks associated with prior art systems.

In contrast to conventional photography, wherein amplitude informationpertaining to the incident light is recorded within the film emulsion, ahologram contains a record of both amplitude and phase information. Whenthe hologram is replayed using the same wavelength of light used tocreate the hologram, the light emanating from the film continues topropagate just as it did when it was "frozen" within the film, with itsphase and amplitude information substantially intact. The mechanism bywhich the amplitude and phase information is recorded, however, is notwidely understood.

As discussed above, the reference beam and object beam, in accordancewith the present invention, are of the same wavelength and polarizationstate at the surface of film 319. The interaction between these two wavefronts creates a standing (static) wave front, which extends through thethickness of the emulsion. At points within the emulsion where theobject and reference beam constructively interact, a higher energy levelis present than would be present for either beam independently. Atpoints within the emulsion where the reference and object beamdestructively interact, an energy level exists which is less than theenergy level exhibited by at least one of the beams. Moreover, theinstantaneous amplitude of each beam at the point of interaction isdefined by the product of the peak amplitude of the beam and the cosineof its phase at that point. Thus, while holographers speak of recordingthe amplitude and phase information of a wave, in practical effect thephase information is "recorded" by virtue of the fact that theinstantaneous amplitude of a wave at a particular point is a function ofthe phase at that point. By recording the instantaneous amplitude andphase of the static interference pattern between the reference andobject beams within the three-dimensional emulsion, a "three-dimensionalpicture" of the object as viewed from the plane of film 319 is recorded.Since this record contains amplitude and phase information, athree-dimensional image is recreated when the hologram is replayed.

After every data slice comprising a data set is recorded onto film 319in the foregoing manner, film 319 is removed from film holder 318 forprocessing.

As discussed above, the photographic emulsion employed in the presentinvention comprises a large number of silver halide crystals suspendedin a gelatinous emulsion. While any suitable photosensitive element maybe employed in this context, silver halide crystals are generally on theorder of 1,000 times more sensitive to light than other knownphotosensitive elements. The resulting short exposure time for silverhalide renders it extremely compatible with holographic applications,wherein spurious vibrations can severely erode the quality of theholograms. By keeping exposure times short in duration for a given laserpower, the effects of vibration may be minimized.

As also discussed above, a hologram corresponding to each of a pluralityof data slices is sequentially encoded onto film 319. After every slicecomprising a particular data set has been recorded onto the film, thefilm is removed from camera assembly 300 for processing. Beforediscussing the particular processing steps in detail, it is helpful tounderstand the photographic function of silver halide crystals.

In conventional photography, just as in amplitude holography, a silverhalide crystal which is exposed to a threshold energy level for athreshold exposure time becomes a latent silver halide grain. Uponsubsequent immersion in a developer, the latent silver halide grains areconverted to silver crystals. In this regard, it is important to notethat a particular silver halide grain carries only binary data; that is,it is either converted to a silver crystal or it remains a silver halidegrain throughout the process. Depending on the processing techniquesemployed, a silver halide grain may ultimately correspond to a darkregion and a silver crystal to a light region, or vice versa. In anyevent, a particular silver halide grain is either converted to silver orleft intact and, hence, it is either "on" (logic hi) or "off" (logiclow) in the finished product.

In conventional photography as well as in amplitude holography, theexposed film is immersed in a developing solution (the developer) whichconverts the latent silver halide grains into silver crystals, but whichhas a negligible affect on the unexposed silver halide grains. Thedeveloped film is then immersed in a fixer which removes the unexposedsilver halide grains, leaving clear emulsion in the unexposed regions ofthe film, and silver crystals in the emulsion in the exposed areas ofthe film. Those skilled in the art will appreciate that the convertedsilver crystals, however, have a black appearance and, hence, tend toabsorb or scatter light, decreasing the efficiency of the resultinghologram.

In phase holography, on the other hand, the exposed film is bleached toremove the opaque converted silver, leaving the unexposed silver halidegrains intact. Thus, after bleaching, the film comprises regions of puregelatinous emulsion comprising neither silver nor silver halide(corresponding to the exposed regions), and a gelatinous emulsioncomprising silver halide (corresponding to the unexposed regions). Phaseholography is predicated on inter alia, the fact that the gelatincontaining silver has a very different refractive index than the puregelatin and, hence, will diffract light passing therethrough in acorrespondingly different manner.

The resulting bleached film thus exhibits fringe patterns comprisingalternating lines of high and low refractive indices. However, neithermaterial comprises opaque silver crystals, so that a substantiallyinsignificant amount of the light used to replay the hologram isabsorbed by the hologram, as opposed to amplitude holographic techniqueswherein the opaque silver crystals absorb or scatter a substantialamount of the light.

More particularly, the present invention contemplates a six-stageprocessing scheme, for example, performed on a Hope RA2016Vphotoprocessor manufactured by Hope Industries of Willow Grove, Pa.

In stage 1, the film is developed in an aqueous developer to convert thelatent silver halide grains to silver crystals, which may be made bymixing, in an aqueous solution (e.g., 1800 ml) of distilled water,ascorbic acid (e.g., 30.0 g), sodium carbonate (e.g., 40.0 g), sodiumhydroxide (e.g., 12.0 g), sodium bromide (e.g., 1.9 g), phenidone (e.g.,0.6 g), and thereafter adding distilled water resulting a 2 literdeveloping solution.

In stage 2, the film is washed to halt the development process of stage1.

Stage 3 involves immersing the film in an 8 liter bleach solutioncomprising distilled water (e.g., 7200.0 ml), sodium dichromate (e.g.,19.0 g), and sulfuric acid (e.g., 24.0 ml). Stage 3 removes thedeveloped silver crystals from the emulsion.

Stage 4 involves washing the film to remove the stage 3 bleach.

Stage 5 involves immersing the film in a 1 liter stabilizing solutioncomprising distilled water (e.g., 50.0 ml), potassium iodide (e.g., 2.5g), and Kodak PHOTO-FLO (e.g., 5.0 ml). The stabilizing stagedesensitizes the remaining silver halide grains to enhance long-termstability against subsequent exposure.

In stage 6, the film is dried in a conventional hot-air drying stage.Stage 6 is suitably performed at 100 degrees fahrenheit; stages 1 and 3are performed at 86 degrees fahrenheit; and the remaining stages may beperformed at ambient temperature.

With momentary reference to FIGS. 12A and 12B, the alternating high andlow refractive index lines of the phase holograms, produced inaccordance with the present invention, are illustrated as black andwhite regions. When the replay beam (PB) illuminates the hologram, thehigher density regions diffract the incoming light differently than thelow density regions, resulting in a bright, diffuse image, as viewed byobserver 32. Although FIG. 12B schematically illustrates the replaymechanism as a reflection phenomenon, the present inventor hasdetermined that the precise replay mechanism is actually a phenomenonrooted in wave mechanics, such that the light actually "bends" aroundthe various fringe surfaces, rather than literally being reflected offthe fringes.

Upon completion of the processing of film 319, the resulting masterhologram may be used to create one or more copies.

In accordance with one aspect of the invention, it may be desirable toproduce a copy of the master hologram and to replay the copy whenobserving the hologram, rather than to replay and observe the masterhologram directly. With reference to FIG. 10, FIG. 10A depicts acollimated replay beam PB replaying a master hologram, with beam PBbeing directed at the film from the same direction as the collimatedreference beam used to create the hologram (H1). This is referred to asorthoscopic reconstruction. This is consistent with the layout in FIG.3, wherein the data slices, corresponding to respective images 1002 inFIG. 10, were also illuminated onto the film from the same side of thefilm as the reference beam. However, when observed by an observer 1004,the reconstructed images appear to be on the opposite side of the filmfrom the observer. Although the reconstructed images 1002 are notliterally behind hologram H1, they appear to be so just in the same wayan object viewed when facing a mirror appears to be behind the mirror.

With momentary reference to FIG. 10B, hologram H1 is inverted and againreplayed with the replay beam PB. In this configuration, known aspseudoscopic reconstruction, the images 1002 appear to the observer asbeing between the observer and the film being replayed. When masterhologram H1 is copied using copy assembly 900, the pseudoscopicreconstruction set forth in FIG. 10B is essentially reconstructed,wherein the master hologram is shown as H1, and a holographic filmcorresponding to the copy hologram is positioned within the images 1002in a plane P. The assembly shown in FIG. 10B illustrates the copy film(plane P) as being centered within the images 1002, thereby yielding acopy hologram which, when replayed, would appear to have half of thethree-dimensional image projecting forward from the film and half thethree-dimensional image projected back behind the film. However, inaccordance with an alternate embodiment of the present invention, thecopy assembly may be configured such that plane P assumes any desiredposition with respect to the data set, such that any correspondingportion of the three-dimensional image may extend out from or into theplane in which the copy film is mounted.

COPY ASSEMBLY

Referring now to FIG. 9, copy assembly 900 is suitably mounted to atable 904 in much the same way camera assembly 3 is mounted to table 304as described in conjunction with FIG. 3. Copy assembly 900 suitablycomprises a laser source 824, respective mirrors 810, 812, 820, and 850,a beam splitting cube 818, a wave plate 816, respective beam expanders813 and 821, respective collimating lenses 830 and 832, a master filmholder 834 having respective legs 836A and 836B, and a copy film holder838 having a front surface 840 configured to securely hold copy filmsubstrate H2 in place.

Film holder 838 and, if desired, respective film holders 834 and 318 aresuitably equipped with vacuum equipment, for example, vacuum line 842,for drawing a vacuum between the film and the film holder to therebysecurely hold the film in place. By ensuring intimate contact betweenthe film and the holder, the effects of vibration and other spuriousfilm movements which can adversely impact the interference fringepatterns recorded therein may be substantially reduced.

Film holders 838 and 318 desirably comprise an opaque, non-reflective(e.g., black) surface to minimize unwanted reflected light therefrom.Film holder 834, on the other hand, necessarily comprises a transparentsurface inasmuch as the object beam must pass therethrough on its way tofilm holder 838. Accordingly, the opaque film holders, may, if desired,comprise a vacuum surface so that the film held thereby is securelyvacuum-secured across the entire vacuum surface. Film holder 834, on theother hand, being transparent, suitably comprises a perimeter channelwherein the corresponding perimeter of the film held thereby is retainedin the holder by a perimeter vacuum channel. A glass or othertransparent surface may be conveniently disposed within the perimeter ofthe channel, and a roller employed to remove any air which may betrapped between the film and the glass surface. Although a preferredembodiment of the present invention employs the foregoing vacuum filmholding techniques, any mechanism for securely holding the film may beconveniently used in the context of the present invention, including theuse of an electrostatic film holder; a pair of opposing glass plateswherein the film is tightly sandwiched therebetween; the use of asuitable mechanism for gripping the perimeter of the film andmaintaining surface tension thereacross; or the use of an air tightcell, wherein compressed air may be maintained within the cell tosecurely hold the film against one surface of the air tight chamber, thechamber further including a bleed hole disposed on the surface of thecell against which the film is held from which the compressed air mayescape.

With continued reference to FIG. 9, laser source 824 is suitably similarto laser 302, and suitably produces laser light of the same wavelengthas that used to create the master hologram (e.g., 514.5 nm).Alternatively, a laser source for producing the copy may employ adifferent, yet predetermined, wavelength of light, provided the anglethat the reference beam illuminates film H1 is varied in accordance withsuch wavelength. Those skilled in the art will appreciate that thewavelength of the reference beam (λ) illuminating hologram H1 isproportional to the sine of its incident angle, i.e. λ=K sin θ.Moreover, by manipulating the processing parameters to either shrink orswell the emulsion, the relationship between the wavelength and theincident angle can be further adjusted in accordance with therelationship λ=K sin θ.

A source beam 825 from laser 824 is reflected off mirror 812 through awave plate 816 and into cube 818. Variable wave plate 816 and cube 818function analogously to beam splitting assembly 310 discussed above inconjunction with FIG. 3. Indeed, in a preferred embodiment of thepresent invention, a beam splitting assembly nearly identical to beamsplitter 310 is used in copy system 900 in lieu of wave plate 816 andcube 818; however, for the sake of clarity, the beam splitting apparatusis schematically represented as cube 818 and wave plate 816 in FIG. 9.

Beam splitting cube 818 splits source beam 825 into an S polarizedobject beam 806 and a P polarized reference beam 852. Object beam 806passes through a wave plate 814 which converts beam 806 to a P polarizedbeam, which then passes through a beam expanding assembly 813 includinga pin-hole (not shown); reference beam 852 passes through a similar beamexpander 821. Respective beam expanding assemblies 813 and 821 aresimilar in structure and function to beam expanding assembly 314discussed above in conjunction with FIG. 3.

Object beam 806 emerges from beam expander 813 as a divergent beam whichis reflected off mirror 850 and collimized by lens 832. Reference beam852 is reflected off mirror 820 and collimized by lens 830. Note thatvirtual beams 802 and 856 do not exist in reality, but are merelyillustrated in FIG. 9 to indicate the apparent source of the object andreference beams, respectively. Note also that object beam 806 andreference beam 852 are both pure P polarized.

The master hologram produced by camera assembly 300 and discussed aboveis mounted in a transparent film holder 834 and referred to in FIG. 9 asH1. A second film H2, suitably identical in structure to film substrate319 prior to exposure, is placed in film holder 838. Object beam 806 iscast onto master hologram H1 at the Brewster's angle associated withfilm H1 (approximately 56°).

With momentary reference to FIG. 12B, hologram H1 embodies fringepatterns which diffract incident light as a function of incidentwavelength. Since hologram H1 was produced with light having the samewavelength as monochromatic object beam 806, we expect hologram H1 todiffract the object beam by the same amount. Hence, object beam 806emerges from hologram H1 after being diffracted by an average angle Kand strikes film surface 840 of film H2. Reference beam 852 is directedat substrate H2 at any convenient angle, e.g., Brewster's angle(approximately 56°).

Film substrate H2 records the standing wave pattern produced by objectbeam 806 and reference beam 852 in the same manner as described above inconnection with film 319 in the context of FIGS. 3, 4, and 12A and 12B.More particularly, the plurality of images corresponding to each dataslice within a data set are simultaneously recorded onto film H2. Theamplitude and phase information corresponding to each date slice isaccurately recorded on film H2 as that amplitude and phase informationexists within the plane defined by film H2. When copy hologram H2 issubsequently replayed, as discussed in greater detail below, the imagecorresponding to each data slice, with its amplitude and phaseinformation intact, accurately recreates the three-dimensional physicalsystem defined by the data set.

In the preferred embodiment discussed herein, master holograms H1 areproduced on a camera assembly 300, and copy holograms H2 are produced ona copy assembly 900. In an alternate embodiment of the presentinvention, these two systems may be conveniently combined as desired.For example, film holder 318 in FIG. 3 may be replaced with film holder834 from FIG. 9, with a subsequent H2 film holder disposed such that theobject beam is transmitted through film holder 834 onto the new H2 filmholder. In this way, the relationship between film holders H1 and H2(FIG. 9) would be substantially replicated in the hybrid system. Tocomplete the assembly, an additional reference beam is configured tostrike the new H2 film holder at Brewster's angle. As altered in theforegoing manner, the system can effectively produce master hologramsand copies on the same rig. More particularly, the master hologram isproduced in the manner described in conjunction with FIG. 3 and, ratherthan utilizing a separate copy rig, the master hologram may simply beremoved from its film holder, inverted, and utilized to create a copyhologram. Of course, the original object beam would be shunted, andreplaced by a newly added reference beam configured to illuminate newlyadded film holder H2.

As also discussed above, the present invention contemplates, for a dataset comprising N slices, recording N individual, relatively weakholograms onto a single film substrate. To a first approximation, eachof the N slices will consume (convert) approximately 1/N of the silverhalide grains consumed during exposure.

As a starting point, the total quantity of photosensitive elementswithin a film substrate may be inferred by sequentially exposing thefilm, in a conventional photographic manner, to a known intensity oflight and graphing the extent to which silver halide grains areconverted to silver grains as a function of applied energy (intensitymultiplied by time). With particular reference to FIG. 2A, thewell-known HD curve for four exemplary film samples illustrate theeffect of exposing film to a predetermined intensity of light over time.At various time intervals, the extent to which the film is fogged, i.e.the extent to which silver halide grains are converted to silver grains,is measured by simply exposing the film to a beam of known intensity,developing the film, and measuring the amount of light which passesthrough the film as a function of incident light. Although typical HDcurves are nonlinear, they may nonetheless be used in the context of thepresent invention to ascertain various levels of fog as a function ofapplied energy.

In accordance with the present invention, the HD curve for a particularfilm (generally supplied by the film manufacturer) is used to determinethe amount of light, expressed in microjoules per square cm, necessaryto prefog the film to a predetermined level, for example, to 10% of thefilm's total fog capacity as determined by the HD curve. Afterprefogging the film to a known level, a very faint, plane gratinghologram is recorded onto the film, and the diffraction efficiency ofthe grating measured. Thereafter, a different piece of film from thesame lot of film is prefogged to a higher level, for example to 20% ofits total fog capacity based on its HD curve, and the same fainthologram superimposed on the fogged film. The diffraction efficiency ofthe faint hologram is again measured, and the process repeated forvarious fog levels. The diffraction efficiency of the grating for eachfog level should be essentially a function of the pre-fog levelsinasmuch as the prefogging is wholly random and does not produce fringepatterns of any kind.

Referring now to FIG. 2B, a graph of diffraction efficiency as afunction of fog level (bias energy) is shown for a particular lot offilm. Note that the curve in FIG. 2 extends until the film isholographically saturated, that is, until a level of prefog is reachedat which the diffraction efficiency of subsequent faint hologramsreaches a predetermined minimum value. The area under the curve in FIG.2 corresponds to the total energy applied to the film until itsdiffraction efficiency is saturated. In the present context, this energyis equivalent to the product of the intensity of the incident light andthe total time of exposure.

For a particular film lot, the area under the curve in FIG. 2Beffectively characterizes the film in terms of its multiple exposureholographic exposure capacity. For a data set comprising N slices, thearea under the curve may be conveniently divided into N equal amounts,such that each data slice may consume 1/N of the total energy under thecurve. Recalling that the energy for a particular slice is equal to theproduct of the intensity of the incident light and time of exposure, andfurther recalling that the intensity of the incident light (e.g., objectbeam) is determined for each slice in the manner described below inconnection with the beam ratio determination, the time of exposure forevery slice may be conveniently determined.

In accordance with a further aspect of the present invention, each lotof film may be conveniently coded with data corresponding to thatrepresented in FIG. 2B. Analogously, most conventional 35 mm film isencoded with certain information regarding the film, for example, datarelating to the exposure characteristics of the film. In a similar way,the information pertaining to the diffraction efficiency curve shown inFIG. 2B may be conveniently appended to each piece of holographic filmfor use in the present invention, for example by applying to the film orto the packaging therefor. The computer (not shown) used to controlcamera assembly 300 may be conveniently configured to read the dataimprinted on the film, and may thereafter use this data to compute theexposure time for each data slice in the manner described herein. Asstated above, the relative intensities of the reference beam to theobject beam at the film plane is known as the beam ratio. Knownholographic techniques tend to define beam ratio without reference to apolarization state; however, a more meaningful definition of the term,particularly in the context of the present invention, surrounds therelative intensities of the reference and object beams (at the filmplane) at a particular common polarization state, i.e. either a common Ppolarization state or a common S polarization state.

For purposes of understanding the role of beam ratio in the presentinvention, it is helpful to point out that holography may beconveniently divided into display holography, in which the hologram isintended to show a three-dimensional image of a selected object, andHolographic Optical Elements (HOE) in which a basic holographic fringepattern is recorded on a film which thereafter functions as an opticalelement having well-defined properties, for example, as a lens, mirror,prism, or the like.

HOEs are formed with simple directional beams leading to simplerepetitive fringe patterns which tend to dominate weak secondary fringeswhich are also formed by scattered and reflected light within theemulsion. Since the secondary fringe patterns are typically ignored tothe first approximation, conventional holographic theory states that toachieve the strongest interference between the two beams, a beam ratioof one should be employed.

In display holography, on the other hand, while the reference beam isstill a simple directional beam, the object beam can be extremelycomplex, having intensity and direction variations imposed by theobject. In addition, objects typically exhibit any number of brightspots which diffuse light at fairly high intensities. The resultingfringe pattern is extremely complex, bearing no simple relationship tothe object being recorded. Moreover, the bright spots (highlights) onthe object act as secondary reference beams, producing unwanted fringepatterns as they interfere with the reference beam and with each other,resulting in many sets of noise fringes, effectively reducing therelative strength of the primary fringe pattern. The resulting"intermodulation", noise (also referred to as self-referencing noise)causes an unacceptable loss of image quality unless it is suppressed.

Conventional holographic theory states that intermodulation noise may besuppressed by increasing the relative strength of the reference beam,with respect to the object beam, by selecting a beam ratio in the rangeof three to 30, and most typically between five and eight. This resultsin strong primary fringes and greatly reduced secondary fringes(intermodulation noise). Thus, existing holographic techniques suggestthat, in the context of display holography, a beam ratio higher thanunity and preferably in the range of 5-8:1 substantially reducesintermodulation noise.

The diffraction efficiency of a hologram, i.e. how bright the hologramappears to an observer, also exhibits a maximum at a beam ratio of one.At beam ratios higher than one, the diffraction efficiency falls off,resulting in less bright holograms when replayed. The conventionalwisdom in existing holographic theory, however, states that sinceintermodulation noise falls off faster than diffraction efficiency asthe beam ratio increases, a beam ratio of between 5-8:1 minimizesintermodulation noise (i.e. yields a high signal to noise ratio) whileat the same time producing holograms exhibiting reasonable diffractionefficiency.

In the context of the present invention, a very low beam ratio, on theorder of unity, is desirably employed, resulting in a maximumdiffraction efficiency for each hologram associated with every dataslice in a particular data set. In the context of the present invention,however, intermodulation noise (theoretically maximum at unity beamratio) does not pose a significant problem as compared to conventionaldisplay holography. More particularly, recall that intermodulation noisein conventional holography results from, inter alia, bright spotsassociated with the objects. In the present invention, the "objects"correspond to a two-dimensional, windowed, gamma-corrected (discussedbelow) data slice. Thus, the very nature of the data employed in thecontext of the present invention results in inherently lowintermodulation noise, thus permitting the use of a unity beam ratio andpermitting maximum diffraction efficiency and very high signal to noiseratio images. Moreover, the selection of a unity beam ratio for eachslice in a data set may be accomplished quickly and efficiently in thecontext of a preferred embodiment of the present invention.

More particularly, variable wave plate 404 may be calibrated by placinga photo diode in the path of the reference beam near film 319 whileshunting the object beam, and vice versa. As the applied voltage to waveplate 404 is ramped up at predetermined increments from zero to amaximum value, the intensity of the reference beam may be determined asa function of input voltage. Since the intensity of the reference beam,plus the intensity of the object beam (before a data slice isincorporated into the object beam) is approximately equal to theintensity of their common source beam and the intensity of the commonsource beam is readily ascertainable, the pure object beam intensity asa function of voltage applied to wave plate 404 may also be convenientlyderived. It remains to determine the proper input voltage to wave plate404 to arrive at a unity beam ratio for a particular slice.

At a fundamental level, each data slice comprises a known number of"pixels" (although not literally so after having passed through imagingassembly 328), each pixel having a known grey level value. Thus, eachdata slice may be assigned a brightness value, for example, as a percentof pure white. Thus, the particular voltage level required to obtain aunity beam ratio for a particular data slice having a known brightnessvalue may be conveniently determined by selecting the unique voltagevalue corresponding to a pure object beam intensity value which, whenmultiplied by the brightness value, is equal to the reference beamintensity value for the same voltage level. This computation may bequickly and efficiently carried out by a conventional computerprogrammed in accordance with the relationships set forth herein.

Accordingly, each data slice has associated therewith a voltage valuecorresponding to the input voltage to wave plate 404 required to achievea unity beam ratio.

In accordance with another aspect of the present invention, each dataslice comprising a data set may be further prepared subsequent to thewindowing procedures set forth above. In particular, imaging assembly328 generates an image comprising various brightness levels (greylevels) in accordance with data values applied to CRT 444. However, itis known that conventional CRTs and conventional light valves do notnecessarily project images having brightness levels which linearlycorrespond to the data driving the image. Moreover, human perception ofgrey levels is not necessarily linear. For example, while an imagehaving an arbitrary brightness value of 100 may look twice as bright asan image having a brightness value of 50, an image may require abrightness level of 200 to appear twice as bright as the image having abrightness value of 100.

Because human visual systems generally perceive brightness as anexponential function, and CRTs and light valves produce images havingbrightnesses which are neither linearly nor exponentially related to thelevels of the data driving the images, it is desirable to perform agamma correction on the data slices after they have been windowed, i.e.after they have been adjusted at a gross level for brightness andcontrast levels. By gamma correcting the windowed data, the grey levelsactually observed are evenly distributed in terms of their perceptualdifferences.

In accordance with a preferred embodiment of the present invention, agamma lookup table is created by displaying a series of predeterminedgrey level values with imaging assembly 328. A photo diode (not shown)is suitably placed in the path of the output of imaging assembly 328 tomeasure the actual brightness level corresponding to a known data value.A series of measurements are then taken for different brightness levelscorresponding to different grey level data values, and a gamma lookuptable is constructed for the range of grey values exhibited by aparticular data set. Depending on the degree of precision desired, anynumber of grey level values may be measured with the photo diode,allowing for computer interpolation of brightness levels for grey valueswhich are not measured optically.

Using the gamma lookup table, the data corresponding to each data sliceis translated so that the brightness steps of equal value in the datacorrespond to visually equivalent changes in the projected image, asmeasured by the photo diode during creation of the lookup table.Moreover, light valve 442, when used in conjunction with wave plate 463as discussed in the context of FIGS. 7 and 8, is typically capable ofproducing a blackest black image on the order of about 2000 times asfaint as the brightest white image. This level of contrast range issimply unnecessary in view of the fact that the human visual system canonly distinguish within the range of 50 to 100 grey levels within asingle data slice. Thus, the maximum desired contrast ratio (i.e. thebrightness level of the blackest region on a slice divided by thebrightness level of the brightest white region on a slice) is desirablyin the range of 100-200:1, allowing for flexibility at either end of thebrightness scale. Since the contrast ratio of a particular slice is thuson the order of one-tenth the available contrast ratio producible by thelight valve, a further aspect of the gamma correction scheme employed inthe context of the present invention surrounds defining absolute blackas having a brightness level equal to zero. Thereafter, a subjectivedetermination is made that the darkest regions of interest on any slide,i.e. the darkest region that a radiologist would be interested inviewing on a slice, would be termed "nearly black." These nearly blackregions would be mapped to a value which is on the order of 100-200times fainter than pure white. Moreover, any values below the nearlyblack values are desirably clamped to absolute black (zero grey value).These absolute black regions, or super black regions, comprise all ofthe regions of a slice which are darker than the darkest region ofinterest.

An additional gamma correction step employed in the present inventionsurrounds clamping the brightest values. Those skilled in the art willappreciate that conventional CRTs and light valves are often unstable atthe top of the brightness range. More particularly, increasing thebrightness level of data driving an image in any particular CRT/lightvalve combination above the 90% brightness level may yield images havingvery unpredictable brightness levels. Thus, it may be desirous to definethe upper limit of brightness level for a data set to coincide with apredetermined brightness level exhibited by imaging assembly 328, forexample, at 90% of the maximum brightness produced by imaging assembly328. Thus, pure white as reflected in the various data slices willactually correspond to 10% less white than imaging assembly 328 istheoretically capable of producing, thereby avoiding nonlinearities andother instabilities associated with the optical apparatus.

Finally, if any slice is essentially black or contains only irrelevantdata, the slice may be omitted entirely from the final hologram, asdesired.

VIEWING ASSEMBLY

Copy hologram H2 is suitably replayed on a viewing device such as theVOXBOX® viewing apparatus manufactured by VOXEL, Inc. of Laguna Hills,Calif. Certain features of the VOXBOX® viewing apparatus are describedin U.S. Pat. Nos. 4,623,214 and 4,623,215 issued Nov. 18, 1986.

Referring now to FIG. 11, an exemplary viewing apparatus 1102 suitablycomprises a housing 1104 having an internal cavity 1106 disposedtherein, housing 1104 being configured to prevent ambient or room lightfrom entering the viewing device.

Viewing apparatus 1102 further comprises a light source 1108, forexample a spherically irradiating white light source, a baffle 1132, amirror 1134, a Fresnel lens 1110, a diffraction grating 1112, and aVenetian blind 1114 upon which copy hologram H2 is conveniently mounted.Venetian blind 1114 and hologram H2 are schematically illustrated asbeing separated in space from diffraction grating 1112 for clarity; in apreferred embodiment of the device, Fresnel lens 1110 suitably forms aportion of the front surface of housing 1104, diffraction grating 1112forms a thin, planar sheet secured to the surface of lens 1110, andVenetian blind 1114 forms a thin, planar sheet secured to grating 1112.Hologram H2 is suitably removably adhered to Venetian blind 1114 by anyconvenient mechanism, for example by suitable clips, vacuum mechanisms,or any convenient manner which permits hologram H2 to be intimately yetremovably bonded to the surface of Venetian blind 1114.

Fresnel lens 1110 collimates the light produced by light source 1108 anddirects the collimated beam through diffraction grating 1112. Thedesired focal length between source 1108 and lens 1110 will bedetermined by, inter alia, the physical dimensions of lens 1110. Inorder to conserve space and thereby produce a compact viewing box 1102,the light from source 1108 is suitably folded along its path by mirror1134. Since source 1108 may be placed near lens 1110 in order tomaximize space utilization, baffle 1132 may be conveniently disposedintermediate source 1108 and lens 1110, such that only light which isfolded by mirror 1134 strikes 1110. In a preferred embodiment of thepresent invention, the focal length of lens 1110 is approximately 12inches.

Diffraction grating 1112 suitably comprises a holographic opticalelement (HOE), for example one produced by a holographic process similarto that described herein. More particularly, diffraction grating 1112 issuitably manufactured using a reference and an object beam having awavelength and incident angle which corresponds to that used inproducing hologram H2 (here 514.5 nm). As discussed above, therelationship between this angle and wavelength are similarly governed bythe equation λ=K sin θ. In a preferred embodiment, diffraction grating1112 is advantageously a phase hologram.

Diffraction hologram 1112 suitably diffracts the various components ofthe white light incident thereon from source 1108 as a function ofwavelength. More particularly, each wavelength of light will be bent bya unique angle as it travels through diffraction grating 1112. Forexample, the blue component of the white light will bend through anangle P; the higher wavelength green light component is bent at agreater angle Q; and the higher wavelength red light is bent at an angleR. Stated another way, diffraction grating 1112 collimizes eachwavelength at a unique angle with respect to the surface of the grating.Those skilled in the art will appreciate, however, that diffractiongrating 1112 is an imperfect diffractor; thus, only a portion of theincident light is diffracted (e.g., 50%), the remainder of theundiffracted light passes through as collimated white light.

Venetian blind (louvers) 1114 comprises a series of very thin, angledoptical slats which effectively trap the undiffracted white lightpassing through grating 1112. Thus, substantially all of the lightpassing through louvers 1114 passes through at an angle, for example theangle at which the light was diffracted by grating 1112. Of course, acertain amount of light will nonetheless be deflected by the louvers andpass through at various random angles.

Moreover, the geometry of the slats comprising louvers 1114 may beselected to produce a resulting hologram with optimum colorization. Moreparticularly, the slat geometry may be selected so that certainwavelengths pass through louvers 1114 essentially intact (the nominalwave band), whereas wavelengths higher or lower than the nominalwavelength will be clipped by the louvers. Moreover, the geometry of theslats may be selected such that light which passes through grating 1112undiffracted does not pass directly through louvers 1114. Bycoordinating the slat geometry, undiffracted light may be substantiallyattenuated, for example, by causing such undiffracted light to reflect anumber of times (e.g., four) between adjacent slats before reachinghologram H2.

Louvers 1114 suitably comprise a thin, planar light control filmmanufactured by the 3M Company. On one surface, louvers 1114 areslightly convex; moreover, a greasy or waxy substance is apparentlyapplied to this surface by the manufacturer. To avoid damage to thedelicate slats, it may be desirable to adhere the louvers to aprotective surface, for example, an acrylic sheet (not shown). Improperapplication of the "greasy" side of louvers 1114 to an acrylic sheetmay, however, produce a nonuniform contact interface between the twosurfaces, which could produce undesirable optical characteristics.

The present inventor has determined that applying a thin coating of ahigh-lubricity particulate substance (e.g., talc) at this interfacetends to yield a contact surface between the acrylic sheet and thelouvers having improved optical characteristics.

Hologram H2 is illustratively adhered to the surface of louvers 1114.Since hologram H2 is suitably produced using the same wavelength andreference beam angle as was used to produce grating 1112, light passingthrough hologram H2 is bent in accordance with its wavelength.Specifically, blue light is bent at an angle of minus P, green light isbent at an angle of minus Q, and red light is bent at an angle of -R(recall that master hologram H1 was inverted during the production ofcopy hologram H2). Consequently, all wavelengths pass through hologramH2 substantially orthogonally to the plane of lens 1110. As a result, anobserver 1116 may view the reconstructed hologram from a viewpointsubstantially along a line orthogonal to the plane of hologram H2.

By coordinating the wavelength-selective diffraction capacity ofdiffraction grating 1112 with the wavelength-selective diffractionproperties of hologram H2, substantially all of the light diffracted bydiffraction grating 1112 may be used to illuminate the hologram. Thus,even the use of a relatively inefficient diffraction grating 1112produces a relatively bright holographic image. Moreover, theholographic image is not unnecessarily cluttered by spurious white lightwhich is not diffracted by grating 1112, in as much as a substantialamount of this spurious light will be blocked by louvers 1114.

Moreover, by mounting the thin, planar hologram, louvers, anddiffraction grating on the surface of a lens which forms a portion ofthe viewing apparatus, the replay beam used to illuminate the hologramis substantially exclusively limited to the collimized light from source1108; that is, spurious noncollimated light is prevented from strikingthe rear surface (right-hand side in FIG. 11) of hologram H2.

When a hologram (H2), produced in accordance with the present invention,is mounted on box 1102, a three-dimensional representation of the objectmay be seen, affording the viewer full parallax and perspectives fromall viewpoints. The present inventor has further determined that thehologram may be removed from the viewbox, inverted, and placed back onthe viewbox. The inverted hologram contains all of the same data as thenoninverted view of the same hologram, except that the observer islooking at the hologram from the opposite direction; that is, points onthe hologram which previously were furthest away from the observer arenow closest to the observer, and vice versa. This feature may beparticularly useful to physicians when mapping out a proposed surgicalprocedure, for example, by allowing the physician to assess the variouspros and cons of operating on a body part from one direction or theother.

The present inventor has also determined that two or more holograms maybe simultaneously viewed on the same viewbox, simply by placing onehologram on top of the other hologram. This may be particularlysignificant in circumstances where, for example, the first hologramcomprises a body part (e.g., hip) which is to be replaced, and thesecond hologram comprises the prosthetic replacement device. Thephysician may thus view the proposed device in proper context, i.e. asthe device would be implanted in the three-dimensional space within thepatient.

The present inventor has also observed that very faint patterns of lightand dark rings are occasionally visible when viewing a hologram inaccordance with the present invention. More particularly, these ringsappear to be a great distance behind the hologram when viewed. Thepresent inventor theorizes that these rings constitute an interferogram,which results from taking a "hologram" of diffusing screen 472 alongwith each data slice. To overcome this problem, diffuser 472 may beshifted slightly (e.g., ten millimeters) within its own plane after eachdata slice is recorded. In this way, the image corresponding to eachdata slice is still projected onto film 319 as described herein, yet aslightly different portion of diffuser 472 is projected for each dataslice, thereby avoiding projecting the same pattern attributable todiffuser 472 for each data slice.

It is also possible to add textual or graphical material, for example toone or more data slices, thus permitting the resulting hologram of thedata set to reflect this textual or graphic material. Such material maycomprise identification data (e.g., patient name, model or serial numberof the object being recorded), or may comprise pure graphicalinformation (arrows, symbols, and the like).

In accordance with another aspect of the present invention, it may beefficient to window only a portion of the data slices and nonethelessachieve satisfactory contrast and shading. For example, for a 100-slicedata set, it may be possible to manually window every tenth data slice,for example, and through the use of computerized interpolationtechniques, automatically window the interstitial data slices.

In accordance with a further aspect of the present invention, it ispossible to select the film plane among the various data slice planescomprising the data set. More particularly, each data slice within adata set occupies its own unique plane. In accordance with the preferredembodiment of the present invention, track assembly 334 is moved forwardor backward such that the data slice which is centered within the volumeof the data set corresponds to the data slice centered within the lengthof travel of track assembly 334. The relative position of imagingassembly 328 and film 319 may be varied, however, so that the plane offilm 319 is located nearer to one end of the data set or the other, asdesired. The resulting hologram H2 will thus appear to have a greater orlesser portion of the holographic image projected into or out of thescreen upon which the hologram is observed, depending on the positionthat the film plane has been selected to cut through the data set.

In accordance with a further aspect of the invention, a plurality ofdifferent holograms may be displayed on a single sheet. For example, ahologram of a body part before surgery may be displayed on the upperportion of a film, with the lower portion of the film being divided intotwo quadrants, one containing a hologram of the same body part aftersurgery from a first perspective, and the other portion containing aview of the same body part after surgery from another perspective. Theseand other holographic compositions may be suitably employed tofacilitate efficient diagnostic analysis.

In accordance with a further aspect of the present invention, the entirebeam path is advantageously enclosed within black tubing or black boxes,as appropriate. This minimizes the presence of undesirable reflections.Moreover, the entire process of making master and copy holograms isadvantageously carried out in a room or other enclosure which is devoidof spurious light which could contact any film surface. Alternatively,the path traveled by any of the beams in the context of the presentinvention may be replaced with fiber optic cable. By proper selection ofthe fiber optic cable, the polarization and Transverse ElectromagneticMode (TEM) of the light traveling through the cable is preserved. Use offiber optic cable permits the system to be highly compressed, andfurther permits the elimination of many of the components of the systementirely (e.g., mirrors). Finally, fiber optic cables may be used tocompensate for a differential path length between the reference beam andthe object beam. Specifically, to the extent the path traveled by one ofthe beams differs from the other, a predetermined length of fiber opticcable may be employed in the path of the beam traveling the shorterlength to compensate for this difference in length and, hence, renderthe two paths equal.

Although the invention has been described herein in conjunction with theappended drawings, those skilled in the art will appreciate that thescope of the invention is not so limited. For example, while the viewbox has been described as being rectangular, those skilled in the artwill appreciate that any suitable mechanical configuration whichconveniently houses the various components of the viewing apparatus willsuffice. Moreover, although the camera and copy assemblies areillustrated as separate systems, they may suitably be combined into asingle system.

These and other modifications in the selection, design, and arrangementof the various components and steps discussed herein may be made withoutdeparting from the spirit of the invention as set forth in the appendedclaims.

I claim:
 1. A method for making a composite hologram, comprising thesteps of:pre-processing data comprising two-dimensional images;providing a thin, substantially planar, photo-sensitive substrate havinga first surface; providing a reference beam at a first angle withrespect to said first surface and an object beam at a second angle withrespect to said first surface; consecutively illuminating a plurality oftwo-dimensional images by said object beam while maintaining said objectbeam at said second angle with respect to said first surface, whereinsaid plurality of images comprises at least twenty images; consecutivelyexposing said substrate to said two-dimensional images and saidreference beam to superimpose a plurality of holograms corresponding tosaid two-dimensional images onto said substrate, while maintaining saidobject beam at said second angle with respect to said first surface; andphoto-processing said substrate to develop said plurality of holograms.2. The method of claim 1, wherein said pre-processing step includes atleast one of cropping, windowing, compositing, and reformatting saidimages.
 3. A method for making a composite hologram, comprising thesteps of:providing a thin, substantially planar, photo-sensitivesubstrate having a first surface; providing a reference beam at a firstangle with respect to said first surface and an object beam at a secondangle with respect to said first surface; consecutively illuminating aplurality of two-dimensional images by said object beam whilemaintaining said object beam at said second angle with respect to saidfirst surface, wherein said plurality of images comprises at leasttwenty images; consecutively exposing said substrate to saidtwo-dimensional images and said reference beam to superimpose aplurality of holograms corresponding to said two-dimensional images ontosaid substrate, while maintaining said object beam at said second anglewith respect to said first surface, said superimposing step includesprojecting said images with a cathode ray tube (CRT); andphoto-processing said substrate to develop said plurality of holograms.4. A method for making a composite hologram, comprising the stepsof:providing a thin, substantially planar, photo-sensitive substratehaving a first surface; providing a reference beam at a first angle withrespect to said first surface and an object beam at a second angle withrespect to said first surface, polarization of said reference beam is atleast 95% pure; consecutively illuminating a plurality oftwo-dimensional images by said object beam while maintaining said objectbeam at said second angle with respect to said first surface, whereinsaid plurality of images comprises at least twenty images; consecutivelyexposing said substrate to said two-dimensional images and saidreference beam to superimpose a plurality of holograms corresponding tosaid two-dimensional images onto said substrate, while maintaining saidobject beam at said second angle with respect to said first surface; andphoto-processing said substrate to develop said plurality of holograms.5. The method of claim 4, wherein said reference beam is 95% pure Ppolarized.
 6. The method of claim 4, wherein said reference beam is 95%pure S polarized.
 7. A method for making a composite hologram,comprising the steps of:providing a thin, substantially planar,photo-sensitive substrate having a first surface; providing a referencebeam at a first angle with respect to said first surface and an objectbeam at a second angle with respect to said first surface, polarizationof said object beam is at least 95% pure; consecutively illuminating aplurality of two-dimensional images by said object beam whilemaintaining said object beam at said second angle with respect to saidfirst surface, wherein said plurality of images comprises at leasttwenty images; consecutively exposing said substrate to saidtwo-dimensional images and said reference beam to superimpose aplurality of holograms corresponding to said two-dimensional images ontosaid substrate, while maintaining said object beam at said second anglewith respect to said first surface; and photo-processing said substrateto develop said plurality of holograms.
 8. The method of claim 7,wherein said object beam is 95% pure P polarized.
 9. The method of claim7, wherein said object beam is 95% pure S polarized.
 10. A method formaking a composite hologram, comprising the steps of:providing a thin,substantially planar, photo-sensitive substrate having a first surface;providing a reference beam at a first angle with respect to said firstsurface and an object beam at a second angle with respect to said firstsurface; consecutively illuminating a plurality of two-dimensionalimages by said object beam while maintaining said object beam at saidsecond angle with respect to said first surface, wherein said pluralityof images comprises at least twenty images; consecutively exposing saidsubstrate to said two-dimensional images and said reference beam tosuperimpose a plurality of holograms corresponding to saidtwo-dimensional images onto said substrate, while maintaining saidobject beam at said second angle with respect to said first surface; andphoto-processing said substrate to develop said plurality of holograms;and, gauging and quantifying image capacity of said substrate prior tosaid exposing step, said gauging step including applying a predeterminedexposure energy to said substrate, thereafter superimposing a fringepattern at a known exposure energy onto said substrate, and measuringdiffraction of said fringe pattern.
 11. The method of claim 10, whereinsaid energy applying step includes fogging said substrate at a specificintensity for a specific duration.
 12. The method of claim 10, whereinsaid superimposing step comprises superimposing said plurality ofholograms in accordance with said quantified image capacity.